you probably just need to wear the right card, elemental converters.. im just a level 89 mecha.
i can 4 hit garm and kaho on floor 80.. with knuckle boost..

i manage to deal 800-1.4m dmg to OH on low level floors of et..
and im just a level 89 which i dont have enough boost on dex and str stats.

probably u missing the most important thing..
u need to hit the arm cannon first and then hit it with knuckle boost.
u dont need to max arm cannon runes. just the 4 runes with -10% ignore def. with that u can get 40% ignore def whenever u hit arm cannon which is why this is a good chance to equip evil bracer instead of rosa. though im still using rosa for the mean time..

303k contri, 530gm

im using full tiered

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+7doomaxe [2mino] morale 3
cat ear beret ,

nut on head[morale 1] or hockeymask [morale1]
spiked scarf
rosa bracelet[monster card],
+10 staunch armor[as* card] morale 2
+10 staunch mana manteu [raydric]
+6 advance sack teddy shoes [ferus card]
str ring [ultraman] with 3.9% dmg inc
staunch ring [ultraman] with 4% dmg inc

twin katana

guts wing